1. Field of the Invention
The present invention relates to an electrically heated catalyst device.
2. Description of Related Art
In recent years, an electrically heated catalyst (EHC) device has attracted attention as an exhaust gas purifier that purifies exhaust gas discharged from an engine of an automobile or the like. In the EHC, even under a condition in which a temperature of the exhaust gas is low and a catalyst is hard to be activated like just after engine start, it is possible to forcibly activate the catalyst by electrical heating and to raise purification efficiency of the exhaust gas.
In an EHC described in Japanese Patent Application Publication No. 2013-136997 (JP 2013-136997 A), an outer peripheral surface of a columnar carrier having a honeycomb structure and configured to carry a catalyst such as platinum or palladium is provided with a surface electrode extended in an axial direction of the carrier. Comb-shaped wiring lines are connected to the surface electrode, so that a current is supplied thereto. When the current expands in a carrier axial direction on the surface electrode, the whole carrier is electrically heated. This activates the catalyst carried by the carrier, so that unburned HC (hydro carbon), CO (carbon monoxide), NOx (nitrogen oxide), and the like in exhaust gas passing through the carrier are purified by catalytic reaction.
The inventors found the following problem about the electrically heated catalyst device. The abovementioned electrically heated catalyst device has such a problem that repetition (heat cycles) of increasing and decreasing a temperature causes a crack in the carrier, so that a current is hard to flow in some of the wiring lines and the current concentrates on other wiring lines, thereby causing melting.
The inventors searched a cause of the occurrence of cracks in the carrier. FIG. 7 is a graph illustrating temperature changes of a carrier and an electric diffusion layer in an electrically heated catalyst device in a related art. A horizontal axis indicates a time and a vertical axis indicates a temperature. As illustrated in FIG. 7, when the temperature increases (in an ON time of current application to the carrier), a temperature difference between the carrier and the electric diffusion layer formed right on the carrier increases, so that a thermal stress caused therebetween increases. This is presumably because a current easily concentrates on a central part of the electric diffusion layer, so that the central part of the electric diffusion layer is easily heated. Note that the electric diffusion layer is provided between the carrier and a surface electrode in order to expand electricity supplied from wiring lines to an axial direction and a circumferential direction of the carrier. The electric diffusion layer is omitted in JP 2013-136997 A.